1. Field of the Invention
The present invention relates to magnetic disk memories for data processing systems, and particularly to such systems wherein the position of the head arm is indicated by a polyphase or like encoder. More particularly, the present invention relates to a system for selecting the optimum output phase of such encoder for use in track following.
2. Prior Art
Magnetic disk memories with concentric data tracks are often used in data processing systems because of their large storage capacity and the speed with which data can be accessed. Large storage capacity is achieved by using many narrow, closely spaced data tracks; units having of the order of 700 tracks per inch are in common use. It will be appreciated that locating a desired track on a disk having such a high track density requires a sophisticated control system.
Advantageously, such control system may utilize a polyphase optical encoder, mechanically coupled to the head arm, to indicate the position of the read/write head with respect to the disk. In a track seeking mode, the desired track is located by counting the number of tracks crossed as the head is moved. This is done by counting the number of half cycles or zero-crossings of one phase output of the encoder.
When the desired track has been reached, the system switches to a track following mode, in which a servo system advantageously servos about the zero-crossing of a particular phase output of the encoder.
If the system is perfectly aligned, such a zero-crossing will occur where the read/write head is exactly over the centerline of a particular track. If the head moves away from the track centerline, say radially inward, the encoder output signal will change in one polarity sense, and if the head moves in the other direction (radially outward) the encoder signal will change in the other polarity sense. With such arrangement, proper track following can be achieved. For thermal correction, a track centerline servo sector may be provided on each track, having radially offset bursts that are read once per revolution. By comparing the relative amplitudes of these bursts, a fine correction can be made to the position servo.
In disc drives of very high density, the track width may often be less than 0.001 inch. It is possible that a very slight mechanical misalignment may be introduced between the head arm and the encoder, as a result of jarring of the disk drive. As a result, the situation might arise that when the head is positioned at the centerline of a track, the output of a particular encoder phase is not near zero (the preferred condition for servoing) but may even be at the crest of the encoder output, in which situation proper servoing cannot be accomplished. For example, if the encoder is at a crest when the head is positioned over the track centerline, movement of the head in either direction (radially inward or radially outward) will cause an encoder output change of the same polarity sense. Proper servoing cannot be carried out.
Thus in a disk drive in which there is factory preselection of which encoder phase is used for the track following servo operation, mechanical jarring may interrupt the head arm/encoder alignment, so as to introduce the problem just discussed. Accordingly, it is an objective of the present invention to provide a system for eliminating such problem. It is a further object of the present invention to provide a system in which, at certain times during disk drive operation (for example, at power-up), a determination is made automatically as to which available encoder phase output or portion thereof achieves best servoing (i.e., has a zero-crossing most closely aligned with the track centerline). The selected phase (or portion thereof) thereafter is used for servoing in the track following mode.
Another objective of the present invention is to provide a system for determining the optimal phase output (or portion thereof) for a head-arm encoder with respect to the use of such signal for servoing in the track following mode. Advantageously this objective is achieved by providing a system which successively uses each phase (or portion thereof) to position the head, and at each such position, uses burst information on the track to confirm whether adequate centering or track alignment has been achieved with that particular phase. The optimal phase (or portion thereof) is selected for subsequent use when servoing in the track following mode.